Hygroscopicity of internally mixed ammonium sulfate and secondary organic aerosol particles formed at low and high relative humidity

Volatile organic matter suspended in the atmosphere such as α-pinene and β-caryophyllene undergoes aging processes and chemical, and photo-oxidation reactions to create secondary organic aerosols (SOAs), which can influence the indirect effect of aerosol particles and the radiative budget. The presence and impact of water vapor and ammonium sulfate (ubiquitous species in the atmosphere) on the hygroscopicity and CCN activity of SOA have not been well characterized. In this research, three water-uptake measurement methods, cavity ring-down spectroscopy (CRD), humidified tandem differential mobility analysis (HTDMA), and cloud condensation nuclei counting (CCNC), were employed to study the hygroscopicity of α-pinene and β-caryophyllene SOAs formed by dark ozonolysis. We observed the changes in water uptake of SOAs in the absence and presence of water vapor at ∼70% RH and ammonium sulfate seeds. Measured hygroscopicity was represented by a single hygroscopicity parameter (κ). Sesquiterpene SOA was observed to be insoluble, hydrophobic, and non-hygroscopic under all experimental conditions and at all initial concentrations, as β-caryophyllene SOA exhibited non-hygroscopic properties with values that were effectively 0. Conversely, monoterpene SOA water uptake is sensitive to increasing RH in the chamber during secondary aerosol formation. Dry and wet seeded monoterpene SOA showed a similar trend of increase despite variability in initial precursor concentrations. We conclude that differences in the viscosity, solubility and hydrophobicity of SOAs may be the primary factor that leads to changes in SOA hygroscopicity formed under low and high relative humidity conditions.